It wasn’t that surprising, really—with the performance pack, the CLS63′s M157 twin turbo V8 cranks out 550 horsepower and 590 lb-ft of torque—enough grunt to drag Trident ballistic missile submarines out of the water, and more raw power than the 2012 GT-R we brought along. But the CLS63 weighs in at 4,277 pounds—448 more than the 530 horsepower, 448 lb-ft GT-R. (Plus, Michael Crenshaw was driving the GT-R most of the time, and I outweigh him by at least 35 pounds.)

Like you’d expect, from a standing start, the GT-R’s all-wheel-drive traction gave it an unassailable advantage over the RWD CLS63. But once the cars got above 50 or 60 miles per hour…Godzilla couldn’t put any more space between itself and the Merc if the Benz was driven by Roland Emmerich hell-bent on catching him for an even shittier sequel to that 1998 disaster. And when we gave the CLS63 an (accidental) half-second head start on one run, it stayed in front all the way to the finish line.

What gives? The GT-R runs a power-to weight ratio of 7.13 pounds per horsepower. The CLS63 carries 7.69 lbs/hp. The Benz’s gears are shorter, but its final drive ratio is longer. I couldn’t dig up a coefficient of drag for the Benz, but I doubt it’s better than the GT-R’s 0.26. So the Mercedes’ near-equivalent performance doesn’t make sense…

The evidence: two separate dynamometer tests performed on cars powered by Performance Package-equipped M157 V8s. The first one was performed by Edmunds Inside Line on a 2011 S63 AMG. In the S63 with PP, AMG rates the M157 at 563 horsepower and 664 lb-ft, so there’s a sign right off that the M157 has more to give than the CLS63 claims. But here’s the kicker: IL’s test found the S63 made 537 horsepower and 611 lb-ft at the wheels. Assuming the usual 15 percent drivetrain power loss, their M157 was making around 630 horsepower and 720 lb-ft of torque at the crank. Yeah.

But while that’s handy background information, it’s hardly definite proof—the Inside Line test was of a different model, and besides, automakers have been known to plug ringers into their press fleets. Ideally, a test of a privately-purchased CLS63 would provide us with a better look—and what do you know, RENNTech happens to have just that.

On their dyno, the CLS63 AMG w/PP laid down 516 horsepower and 545 lb-ft of torque. Again, assuming 15 percent drivetrain loss, this works out to about 610 horsepower and 640 lb-ft of torque at the crankshaft—60 more horsepower and 50 more lb-ft than Mercedes-Benz says.

Assuming the RENNTech car wasn’t an outlier (and the Inside Line test suggests it isn’t) and it serves as a suitable example of the breed, the average CLS63 AMG with Performance Package makes about 11 percent more horsepower and nine percent more torque than its official rating. Is that enough to let a two-plus ton luxury sedan run with a technological marvel of a sports car? My gut says oh, yeah.

So the next time you see a new AMG rolling down the highway, give the driver a salute. Odds are good he doesn’t even know how much power he’s got at his disposal. It’ll be our little secret that we share with the engineers at AMG. And bravo to them (and to Mercedes-Benz) for building cars that promise great things, but deliver even greater things.